1. Field of the Invention
The present invention relates to air-fuel and/or oxygen-fuel burners intended to be fitted into industrial furnaces, especially furnaces for melting, for example, non-ferrous metals (aluminum, copper, lead, etc.), reheat furnaces, annealing furnaces, or furnaces intended for preheating ladles, and in general to burners intended for so-called xe2x80x9clow temperaturexe2x80x9d melting or heating applications. The industrial furnaces of the present invention may be used in applications at a temperature lying within a range extending from approximately 500xc2x0 C. to 1500xc2x0 C.
The invention also relates to the quarl blocks fitted with such a burner.
2. Description of the Related Art
To obtain optimum, uniform and economic heating requires the formation of a stable flame. The emission of nitrogen oxides, which are extremely harmful, also needs to be reduced.
Techniques are already known which aim to stabilize the flame in air-fuel fired operation and/or to reduce emissions of nitrogen oxides.
For example, document EP 0 644 374, which recommends rotating the injected air, describes a complex assembly of metal mechanical parts whose operation within the temperature range indicated above results in relatively rapid wear, requiring the worn parts to be periodically replaced.
Document EP 0 757 205 recommends the injection of a jet of air at an angle which takes this jet away from the first oxygen-fuel combustion zone, thus creating a recirculation zone in a refractory block. The fact that the jet of air is directed away from the jet of oxygen is not generally something which makes it easy to obtain a stable flame. Furthermore, it appears to be difficult to make a burner equipped in this way operate, as required, in oxy-fuel mode or in air-fuel mode.
Document U.S. Pat. No. 4,797,087 recommends the staging of the combustion, namely the creation of a fuel-rich first mixture followed by the injection of a second oxidizer into a second mixing zone. However, the range of working temperatures is markedly higher than that intended by the invention and the subject-matter of that document does not seem to lend itself to purely oxy-fuel operation.
The object of the invention is to remedy the drawbacks of the known techniques and to create an air-fuel/oxy-fuel burner which produces a stable flame, is of simple construction, is reliable, has small dimensions and is capable of working, as required, either in oxy-fuel mode or in virtually wholly air-fuel mode.
For this purpose, the invention relates to an air-fuel or oxy-fuel burner including a pipe having a free end region designed to be housed in a quarl of an industrial furnace, this pipe having at least one fuel feed duct; a sheath surrounding the duct or ducts over part of their length in order to define a first oxidizer feed channel between the sheath and the duct; a sheath flange extending transversely around the sheath; a jacket fitted around the sheath and disposed around at least part of the length of the free end region of the pipe, the sheath designed to be housed in the quarl and to define a second oxidizer feed channel between the jacket and the inner surface of the quarl; and a jacket flange extending transversely around the jacket for securing the jacket to the quarl and the sheath flange being fastened to the jacket flange.
By virtue of this structure, an air-fuel and/or oxy-fuel burner is obtained which is simple to manufacture, occupies little space and is easy to fit.
The burner may furthermore have one or more of the following characteristics:
each fuel feed duct comprises an injector provided at a free end of a rigid conduit, and the actual free end of the injector constitutes the free end of the duct;
the jacket is fitted around the sheath;
the jacket extends beyond the sheath, approximately so as to be in line with the free end of the fuel feed duct or ducts, in order to define, between the jacket and the ducts, an oxygen feed channel extending as far as the free end;
the jacket has a smaller cross section than that of the sheath starting from a region lying slightly beyond the free end of the sheath;
the flange for the jacket carries studs for fastening the flange for the sheath;
the pipe has a longitudinal axis with respect to which the sheath is off-centered in order to leave, between the sheath and the fuel feed duct or ducts, a wider space on one side in order to accommodate a tube for a flame control device and a pilot burner;
the sheath has a part of flared shape and one end of an oxygen feed tube emerges in this flared part;
the flange for the jacket has an opening in which one end of the a feed tube for the second oxidizer emerges;
the velocity of the fuel in the fuel feed duct or ducts lies within a range of approximately 10 m/s to 180 m/s;
oxygen flows in the first oxidizer feed channel at a velocity at most equal to approximately 20 m/s and preferably equal to approximately 8 m/s; and
natural gas flows in the fuel feed duct or ducts with a total flow rate of approximately 50 Sm3/h and at a velocity of approximately 30 m/s and oxygen flows in the first oxidizer feed channel with a flow rate of approximately 100 Sm3/h and at a velocity of approximately 8.5 m/s.
The invention also relates to a quarl block fitted with a burner as defined above, comprising a quarl passing right through the quarl block and having an inlet chamber and an outlet chamber which are joined by a transition zone flared towards the inlet chamber, and the fuel feed duct or ducts of the pipe of the burner penetrate, from the inlet chamber, right into the outlet chamber.
The quarl block may also have one or more of the following characteristics:
in the case in which the outlet chamber is approximately cylindrical, the distance L between the free end of the fuel feed ducts and the end of the quarl located on the internal side of the furnace and the diameter d of the outlet chamber satisfy the relationship 0.3dxe2x89xa6Lxe2x89xa61.8d;
the inlet chamber is off-centered with respect to a longitudinal axis of the pipe and with respect to the outlet chamber and the transition zone, in order to leave a wider space between the inner surface of the quarl and the jacket allowing the introduction of the second oxidizer;
the oxidizer feed channel is designed for an oxygen stream to flow in it, the feed channel for a second oxidizer is designed for a stream of air to flow in it and the fuel feed duct or ducts are designed for a stream of natural gas to flow in them;
oxygen flows in the oxidizer feed channel with a flow rate lying within a range going from approximately 1% to 5% of the total (oxygen, air and fuel) flow rate;
air flows in the feed channel for a second oxidizer at a velocity at most equal to approximately 50 m/s and preferably equal to approximately 30 m/s;
natural gas flows in the fuel feed duct or ducts with a total flow rate of approximately 50 Sm3/h and at a velocity of approximately 30 m/s, oxygen flows in the oxidizer feed channel with a flow rate of approximately 50 Sm3/h and at a velocity of approximately 4.25 m/s and air flows in the feed channel for a second oxidizer with a flow rate of approximately 250 Sm3/h and at a velocity of approximately 13.4 m/s; and
natural gas flows in the fuel feed duct or ducts with a total flow rate of approximately 50 Sm3/h and at a velocity of approximately 30 m/s, oxygen flows in the oxidizer feed channel with a flow rate of approximately 15 Sm3/h and at a velocity of approximately 1.3 m/s and air flows in the feed channel for a second oxidizer with a flow rate of approximately 427 Sm3/h and at a velocity of approximately 23 m/s.
By virtue of these characteristics, it is also possible to obtain a highly stable flame.